At first the ship’s operators said it was piracy. But the attack came from the sky. On July 29th an unmanned drone laden with explosives was flown into the MV Mercer Street (pictured), an oil tanker managed by an Israeli-owned firm, just off the coast of Oman. Two people on board, a British national and a Romanian citizen, were killed.

The strike wasn’t carried out by pirates, said America, Britain and Israel. It was the work of Iran. “This is exactly the reason why we must act now against Iran,” said Benny Gantz, Israel’s defence minister.

Iran quickly denied involvement—while ratcheting up the tension. Saeed Khatibzadeh, a spokesman for the foreign ministry, accused Israel of creating “instability, terror and violence”, and warned that “whoever sows the wind shall reap the whirlwind”.

And so the shadow war between Iran and Israel continues. In recent years several of their vessels have been attacked, with each side blaming the other. Israel has also struck Iranian positions in Syria and is accused of attacking important facilities in Iran, sometimes with cyber-weapons. An operation earlier this year, with conventional explosives planted at the site, did significant damage to Iran’s main uranium-enrichment complex, in Natanz.

The strike on the Mercer Street came just a week before Hassan Rouhani, a pragmatist who reached out to the West, is due to step down as Iran’s president (after the maximum two consecutive terms). Ebrahim Raisi, who won a rigged election in June, will take his place. Mr Raisi, a hardline cleric, has a narrow and suspicious view of the world, and of the West in particular. He rarely travels beyond Iran’s religious seminaries, let alone abroad. “A darker Iran is emerging,” says a foreign observer in Tehran.

If Iran was behind the attack, it was probably the work of the Islamic Revolutionary Guard Corps (IRGC), the country’s most powerful military force. Few Iranian presidents have succeeded in curbing the Guards, whose workings are murky and who often seem to pursue a policy at odds with that of the weak elected government. Mr Raisi is unlikely even to try. If the Guards answer to anyone it is to the supreme leader, Ayatollah Ali Khamenei. Mr Raisi was handpicked by Mr Khamenei and is widely considered his yes-man. “Under the new administration, Iran will be much more assertive,” says Mohammad Marandi, an Iranian academic with close ties to the IRGC.

All of this complicates efforts to revive the nuclear deal signed in 2015 by Iran and six world powers. That agreement, called the Joint Comprehensive Plan of Action (JCPOA), had Iran curb its nuclear programme and agree to rigorous inspections in return for the lifting of some international sanctions. But under President Donald Trump, America then abandoned the deal in 2018. In response, Iran began breaching parts of it.

President Joe Biden promised to return to the deal if Iran came back into compliance. But a sixth round of indirect negotiations to revive the JCPOA ended in June. There was no date set for the resumption of talks. Iran, meanwhile, has continued to steadily expand its nuclear activity. It is enriching uranium beyond the levels required for civilian use and it is no longer co-operating with inspectors.

Nevertheless, America and Britain may want to limit any response to the strike on the Mercer Street in order to keep the JCPOA talks alive. Britain summoned Iran's ambassador over the "unlawful and calous attack”. Iran summoned the British chargé d’affaires in Tehran, as well as Romania's top envoy, to protest against the accusations that Iran was behind the attack.

Israel’s leaders, never fans of the JCPOA, may prefer a more forceful response. “This time the price is going up,” says Amos Yadlin, a former head of Israeli military intelligence.